1. Field of the Invention
This invention relates to a dry etching method employed in the field of production of semiconductor devices. More particularly, it relates to a method for anisotropic etching of a layer of a material being etched without employing a CFC based gas.
2. Description of the Prior Art
As the design rule for the semiconductor devices becomes finer through the submicron level to the quarter micron level, various attempts have been made towards higher resolution and higher anisotropy in a variety of etching processes, such as etching a polysilicon layer, a refractor metal silicide layer or a polycide film for forming an electrode/interconnection layer or silicon trench etching for forming capacitors or isolation.
Heretofore, a gas system mainly consisting of a chlorofluorocarbon (mentioned as CFC hereinafter) based gas, such as CFC 113 (C.sub.2 Cl.sub.3 F.sub.3), has been used for etching layers of these materials. The reason is that, since the CFC-based gas contains both fluorine and chlorine atoms in the molecule, the etching reaction may proceed effectively under both the radical mode and the ion mode, and that, since the carbonaceous compound produced from the CFC-based gas into the gaseous phase is deposited on the etching surface in the form of a carbonaceous polymer, high anisotropy may be achieved simultaneously with sidewall protection.
With a system in which the ion sputtering action can be expected, the layer of the resist material may become a carbon source for forming the carbonaceous polymer.
In contrast to the above attempts towards realizing high anisotropy by acting on the etching gas composition or etching conditions, a so-called low-temperature etching method, according to which etching is carried out while the sample wafer is cooled to a temperature not higher than 0.degree. C., is recently attracting attention. The low temperature etching method is a technique in which, as reported for example in Proceedings of Dry Process symposium, 1988, pages 42 to 49, the sample wafer is maintained at a lower temperature for suppressing the radical reaction in the sidewall section, while the etch rate along the depth is maintained by the ion assist effect, thereby preventing shape defects, such as side etching.
However, the process employing the CFC based gases suffer from various inconveniences.
First, the CFC based gases are responsible for destruction of the ozone layer of the earth and the tendency towards the unusually high atmospheric temperature. Since manufacture and use of the CFC-based gases will be prohibited in the near future, it has become incumbent in the field of dry etching to develop a substitute etching gas and a technology of using the gas.
Second, as the design rule for the semiconductor device is expected to be finer in future, it is feared that more serious consequences may be produced as a result of contamination by a carbonaceous polymer particle derived from the CFC-based gas.
It has also been reported that the carbon contained in the CFC base gas is capable of reducing selectivity of silicon oxide to silicon. If this should occur actually, serious consequences may be produced due to the lowering of the selectivity when etching the polysilicon layer on a thin gate oxide film.
In light of the above, it has been strongly desired not to make use of the CFC based gas.
As long as low temperature etching is concerned, it is usually a process in need of cooling to about -100.degree. C. Since a cooling system capable of developing a high cooling capacity is required, the etching system is increased in size and production costs. On the other hand, the throughput of the etching process is lowered due to the prolonged wafer cooling, and prolonged wafer heating before exposure to the air for preventing dewing. Thus a demand has been raised for a process which may be carried out at a temperature closer to the room temperature.
In addition, when an etching mask is formed by the multi-layer resist process, there is also raised a problem that the sidewall protection film derived from the resist material can hardly be produced. For example, in a three-layer resist process in which a thicker lower resist layer, an intermediate silicon oxide film and a thin upper resist layer are formed step by step, the upper resist layer is patterned by photolithography, the intermediate film is patterned, with the upper resist layer as the mask, and the lower resist layer is etched, with at least the intermediate film as the mask. If the upper resist layer is removed by sputtering during etching of the lower resist layer to expose the intermediate film, there exists no resist material as the source of the carbonaceous polymer on the surface on which ions fall vertically, so that the sidewall protection film cannot be formed. Since the sidewall protection is necessary for suppressing the spontaneous reaction of chlorine and aluminum in the case of, above all, the etching process for etching the layer of the aluminum based material by the chlorine based gas, it becomes impossible with the conventional gas system to realize anisotropic etching of the layer of the aluminum material by the multi-layer resist process.